NEW DELHI  A joint Indian-Chinese team plans to chart remote Himalayan glaciers that scientists fear are rapidly melting because of global warming, threatening the great rivers that give life to one of South Asia's most fertile regions.

The two expeditions, announced Thursday, will take scientists into some of the most remote areas of Tibet to explore the sources of two rivers that provide water for vast agriculture regions that feed nearly a sixth of the world's population.

"The melting of the ice sheets and the glaciers is a crisis in the Himalayas," said H.P.S. Ahluwalia, who runs the Indian Mountaineering Foundation, which is organizing the expedition with China's Institute of Geology and Geophysics.

Scientists believe that increasing global temperatures are causing glaciers -- the planet's largest source of fresh water after polar ice -- to melt.

The short-term result has been flooding, but some fear that over the long term the glaciers will melt entirely and the rivers will run dry for months at a time, fed only by annual rains like the monsoon that sweeps across the subcontinent every summer.

"In three to four decades these rivers that feed more than a billion people in our society and adjoining countries will become seasonal rivers," Ahluwalia said.

Scientists will study the sources of the Sutlej and the Brahmaputra, two rivers which -- like the better known Indus and the Ganges rivers -- flow from the Himalayas into northern India where the fertile plains they feed form the backbone of a society that is still largely agricultural.

All are regarded as life-giving holy rivers -- Hindus even venerate the Ganges as a goddess.

Almost every one of more than 300 large glaciers studied worldwide -- from the Andes in South America to the Himalayas -- is in retreat, international glaciologists reported in October in the journal Geophysical Research Letters.

The dangers faced by Himalayan glaciers have been exacerbated by India's and China's huge populations and fast-growing economies, which rely heavily on coal as an energy source. Burning coal is a major source of the gases scientists blame for warming the Earth.

A recent report from the British government said if no action is taken to stop climate change, average global temperatures will rise by 3.6 degrees to 5.4 degrees within the next 50 years or so, and the Earth will experience several degrees more of warming if emissions continue to grow.

Beginning in September 2007, expedition teams will explore the glaciers around Mount Gang Rinpoche, which is 21,778 feet high, and Mount Loinbo Kangri, at an elevation of 23,277 feet. Neither mountain has been scientifically surveyed in nearly a century.